1. Field of the Invention
The invention relates to a method of selective etching a first material on a substrate with a high selectivity towards a second material.
2. Description of Related Art
Such a selective etching can be used in semiconductor device manufacturing process or semiconductor substrate manufacturing process. Hence said substrate may be a semiconductor wafer.
The process may be used for successful integration of thin monocrystalline semiconductor layers like strained silicon. A thin layer of SiGe can be used as substrate for epitaxial growth of strained silicon. Since Ge has bigger atoms than Si, the lattice constant of SiGe alloys can be adjusted by the Ge content. The typical Ge content is 20-30 atom % for the usage of SiGe as substrate for growing strained silicon. SiGe thereafter is removed, thus having the role of a sacrificial layer. The remaining strained silicon has advanced electrical properties, with electrons and holes flowing faster leading to a 20-30% increase in transistor performance. Strained silicon is also used in combination with the Silicon-On-Insulator (SOI) technology, thus forming strained-SOI. SOI-based chips are using a thin insulation layer (typically silicon oxide) under the monocrystalline silicon surface, thus allowing higher speeds with less electrical loss.
Such a process is described in U.S. Pat. No. 6,603,156 as follows: A SOI structure and a method for its fabrication, in which a strained silicon layer lies directly on an insulator layer, contrary to the prior requirement for strained-Si layers to lie directly on a strain-inducing (e.g., SiGe) layer. The method generally entails the forming a silicon layer on a strain-inducing layer so as to form a multilayer structure, in which the strain-inducing layer has a different lattice constant than silicon so that the silicon layer is strained as a result of the lattice mismatch with the strain-inducing layer. The multilayer structure is then bonded to a substrate so that an insulating layer is between the strained silicon layer and the substrate, and so that the strained silicon layer directly contacts the insulating layer. The strain-inducing layer is then removed to expose a surface of the strained silicon layer and yield a strained silicon-on-insulator structure that comprises the substrate, the insulating layer on the substrate, and the strained silicon layer on the insulating layer. As a result, the method yields a strained silicon-on-insulator (SSOI) structure in which the strain in the silicon layer is maintained by the SOI structure. A therein disclosed method for completely removing the SiGe substrate is by a selective chemical etching process such as HHA (hydrogen peroxide, hydrofluoric acid, acetic acid) etching, which etches the SiGe substrate.
Typical layer thicknesses are 100 nm for SiGe and 20 nm for strained silicon. The sacrificial SiGe layer typically is removed by wet chemical etching. Etching with high selectivities is preferred to keep the strained silicon loss as low as possible.
As described in G. Chang, T. Carns, S. Rhee, K. Wang, J. Electrochem. Soc., Vol. 138, No. 1, 202-4, wet etchants of SiGe typically comprise an oxidizing agent like H2O2 or HNO3 and a silicon complexing agent like HF or NH4OH. To achieve higher selectivities, etchants are diluted with water, or moderators like acetic acid or phosphoric acids are added. High efforts have been made to increase etchrate and selectivity of etching SiGe towards Si or strained Si. Proposed methods for increasing this selectivity comprise adding moderators like acetic acid.